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STANDARD MICROSYSTEMS 




By: Daniel Kllcourse 



INTRODUCTION 



The original ARCNET* star topology networks, those which make use of the LAND (Local Area 
Network Driver - HYC9068) module, are straightforward with respect to cable lengths and the 
number of nodes. Simply stated, the user is allowed a maximum distance of 2000 feet between any 
two devices containing a LAND module. 

Permissible distances become somewhat of a more complicated issue when dealing with the bus 
topology, made possible by the HIT (High Impedance Transceiver - HYC9088) module and worse 
when combining the two types of modules on a single cable. This technical note addresses the 
coaxial cable length vs. number of nodes issue for HIT only topologies, as well as HIT/LAND 
topologies. Twisted Pair issues do not fall within the scope of this technical note. 



DEVICE/MODULE IDENTIFICATION 

Table 1 lists SMC ARCNET products and indicates the type of module used in each product. 



Table 1 



DEVICE 


MODULE 


PC110 


LAND 


PS1 10 


LAND 


Active Hub-01 


LAND (all ports) 


Active Hub-Opt 


LAND (coax ports only) 


PC210 


HIT 


Active Link 


HIT (both ports) 


Active Link-Opt 


HIT (coax port only) 


Active Link-Opt 2 


HIT (coax port only) 


Twisted Pair Link 


HIT (coax port only) 



HIT ONLY TOPOLOGIES 

SMC has specified that eight nodes may be connected to 1 000 feet of RG62 coaxial cable. In fact, 
more nodes may be connected if the cable is shorter and fewer nodes are allowed if the cable is 
longer. A node, for the purposes of this discussion, is defined as any device containing a HIT and 
is connected to the cable. 

A HIT will output a signal 9 dB over and above that required to drive its own receiver and insure 
proper signal to noise ratio for the remaining nodes. Each additional node has an insertion loss of 
0.43 db and RG 62 cable has an attenuation of 0.6 dB per 100 feet at 5 MHz (5 MHz is the 



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3. Since the LAND provides the proper termination impedance, the terminator typically connected 
to the end of the cable in the bus topology must NOT be connected. 

4. Use Table 2 to determine the maximum cable lengths for the number of desired nodes. In a 
mixed HIT/LAND environment, do NOT count devices containing LAND modules as nodes. 

5. A device containing a LAND module must ALWAYS have power applied. If power is lost, the 
cable will no longer be terminated properly and may result in a non-functioning network. 

Once, again, the HIT/LAND Bus topology is NOT recommended but is presented above for those who 
find it necessary to use this topology in their application. 



QUESTIONS AND ANSWERS 

1 . Can a HIT-type device be connected to an existing star configuration which has 2000 feet of 
cable and a LAND-type device at each end? 

A HIT device presents an additional .43 dB loss to the network. This is equivalent to 
approximately 72 feet of RG62 cable. A HIT device may be connected to a cable 1 928 feet long 
(2000 ft. - 72 ft.) which has a LAND device on each end providing the distance between the HIT 
device and the furthest node is no greater than 1 500 feet. 

2. With respect to termination characteristics, how does the HYC9068 (LAND) differ from an 
HYC9088 (HIT) which is terminated externally with a 93Q resistor? 

The HYC9068 (LAND) has a 930. impedance only when power is applied to the device. An 
HYC9068 must have power applied for it to function properly as a terminator. The HYC9088 
(HIT) maintains a high impedance whether power is applied or not. This allows a HIT to be 
placed anywhere on the cable. When a HIT and a 93Q resistor are connected to the end of a 
cable, it is the resistor which provides the actual termination. To summarize, with power 
applied, there is no difference between a LAND and a HIT with a 930 resistor. With power 
removed, a LAND will lose its terminating property while the HIT with a 930 resistor will 
maintain its terminating property. 

3. Can a HIT be connected to a Passive Hub? 

No. Due to their heavy load and short cable length restrictions, the use of Passive Hubs with 
HITS is not allowed. 

4. Can a HIT be used to directly drive Twisted Pair cable? 

Yes. The HYC9088 may directly drive either coaxial or twisted pair cable in a Bus topology. 



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